The present invention relates to a method for manufacturing a storage capacitor of a very large scale integrated (VLSI) semiconductor device. More particularly, the present invention relates to a method for forming the lower electrode of a capacitor to be used for fabricating a 1 Gbit or above dynamic random access memory (DRAM).
As DRAM device densities increase to 64 Mbits and above, the types of capacitors used in these devices have changed and must continue to change to meet a growing need for smaller capacitors with relatively large capacitances. To meet this growing need, conventional capacitors using NO (nitride and oxide) thin films as their dielectric layers have been developed using planar structures, trench structures, stack structures, cylinder structures, and fin structures to increase available capacitance. Of these structures, cylinder and fin capacitor types are limited in terms of economy and reliability due to their extremely complex structures and intricate fabrication methods.
Studies on the use of high-dielectric thin films to overcome problems incurred from the complexity of the required capacitor structure have been ongoing in the United States and Japan for the past ten years. From these studies, perovskite-structured materials such as barium titanium oxide (BaTiO3), lead titanium oxide (PbTiO3), strontium titanium oxide (SrTiO3, or more simply, STO), lead zirconium titanium oxide (Pb(Zr,Ti)O3) and barium strontium titanium oxide ((Ba,Sr)TiO3, or more simply, BST) have attracted interest.
In particular, the very high dielectric constants of STO and BST materials (ranging from 300 to 600) make these materials appropriate for highly-integrated semiconductor capacitors. STO and BST materials allow for the simplification of capacitor processing in VLSI semiconductor device applications such as DRAMs of more than 1 Mbit by allowing the use of capacitors with a simpler physical design. Because of the high dielectric constant of the material used in the dielectric layer, these capacitors can employ simple designs yet still obtain a sufficiently large capacitance.
A study on the application of capacitors having STO thin films as their dielectric films to 64 Mbit DRAMs has recently been conducted, as shown in H. Yamaguchi et al., “Structural and Electrical Characterization of SrTiO3 Thin Films Prepared by Metal Organic Chemical Vapor Deposition,” Japan Journal of Applied Physics, Vol. 32, Part 1, No. 913, pp. 4069-4073, (1993). When using the above dielectric films having high dielectric constants, the general-purpose polysilicon used with conventional NO and Ta2O5 thin films cannot be used as an electrode material. This is attributed to the susceptibility of polysilicon to oxidation during a thin film deposition process or subsequent thermal process because of the presence of a high dielectric film. If a low dielectric oxide layer is formed at the interface between the electrodes and the dielectric layer capacitance rapidly decreases, thus negating the beneficial effects of the high dielectric material.
Accordingly, when fabricating a capacitor with a high dielectric material, a lower electrode, on which a dielectric film is deposited, must be formed of a material which can withstand thermal processing. A noble metal that resists oxidation, such as platinum (Pt), or an oxide material, such as ruthenium oxide (RuO2), have so far been used for the lower electrode when fabricating a capacitor with a high dielectric material. Pt and RuO2 have their own advantages and disadvantages for use in the lower electrode of a high dielectric capacitor.
As is generally known, Pt is difficult to pattern into a storage node since it is a chemically stable metal. Although the possibility of patterning Pt by means of a variety of gases is being explored, the problems of sidewall deposition of an etched object and low etch rate have yet to be solved.
RuO2, in comparison, is easy to etch. However, the leakage current of an STO or BST film deposited on RuO2 is about 100 times larger than if the same film were deposited on a Pt electrode. Despite its ease of etching, this large leakage current makes the use of RuO2 unacceptable.